In recent years, the reduction of carbon dioxide emissions has been sincerely desired in order to address air pollution and global warming. The automotive industry has a growing expectation on the introduction of electric vehicles (EV) and hybrid electric vehicles (HEV) for the reduction of carbon dioxide emissions and has been intensively working on the development of motor-drive secondary cells, which become key to the practical application of these electric vehicles.
As the motor-drive secondary cells, attentions are being given to lithium-ion secondary cells of relatively high theoretical energy. The development of the lithium-ion secondary cells has been rapidly pursued at present. In general, the lithium-ion secondary cell includes a positive electrode, a negative electrode, a separator and electrolyte arranged between the positive and negative electrodes and an outer casing member accommodating therein the positive and negative electrodes, separator and electrolyte. In the lithium-ion secondary cell, for example, lithium cobaltate (LiCoO2) or lithium manganate (LiMn2O4) is applied as a main material of the positive electrode; and graphite is applied as a main material of the negative electrode. Further, a porous polyolefin etc. is used for the separator; lithium hexafluorophosphate is used as the electrolyte; and a laminate film etc. is used for the outer casing member in the lithium-ion secondary cell.
There has conventionally been proposed a non-aqueous secondary cell using a lithium-manganese composite oxide represented by LixMn2−yMAyO4+z (where MA is at least one kind of element selected from the group consisting of Mg, Al, Cr, Fe, Co and Ni; 1<x≤1.2; 0<y≤0.1; and −0.3≤z≤0.3) as a positive electrode active material so as to prevent the elution of manganese from the lithium-manganese composite oxide active material.
It is an essential condition for this non-aqueous secondary cell to use, as a negative electrode active material, a mixture of a carbon material powder in which high-crystalline carbon particles are coated with low-crystalline carbon in the form of a mixture with non-carbon-coated, graphitized meso-carbon microbeads since there is a need to add a large amount (e.g. 10% by weight or more) of binder into the negative electrode due to the bulkiness of the carbon material powder (see Patent Document 1).
The present inventors have however obtained, as a result of research, a new technical finding that the lithium-ion secondary cell increases in resistance in the case of using the lithium-manganese composite oxide partially substituted by magnesium as the positive electrode active material and the mixture of the non-carbon-coated graphite and the carbon-coated graphite as the negative electrode active material.
The present inventors have made further research on such a finding and resultingly found that, although the negative electrode has exposed parts between crystalline layers of the non-carbon-coated graphite (as inlet and outlet for the transfer of lithium ions), these exposed parts are readily clogged by magnesium deposition during long-term use so that the non-carbon-coated graphite becomes deactivated in advance so as to develop nonuniformity of stress in the negative electrode active material layer due to difference in expansion/contraction rate between particles of the coated graphite and non-coated graphite by charge and discharge operation and make it likely that the adhesion of the particles will be broken to cause increase in cell resistance.
The present invention has been established based on these new technical findings. It is an object of the present invention to provide a lithium-ion secondary cell capable of preventing increase in resistance.